The mobile communication technology constantly pursues improvements in quality and quantity to satisfy up-to-date requirements. In the next-generation mobile communication system, a relaying system is used to expand the system coverage, increase the overall throughput, promote the transmission efficiency, and reduce the path loss between a base station and a mobile station, whereby is effectively saved the transmission power of the mobile station and greatly prolonged the run time of batteries of the mobile station. If the relaying systems are appropriately disposed on the boundaries of the base station coverage or the areas having a serious shadowing effect, the base station can provide uniform data rate coverage for users at different locations within the coverage.
A relaying system may work in a DF (Decode-and-Forward) mode or an AF (Amplify-and-Forward) mode. Compared with the DF relaying system, the AF relaying system is free of active elements for decoding. Therefore, the AF relaying system has the advantages of low cost, easy installation and small size.
The OFDM (Orthogonal Frequency Division Multiplexing) technology is an efficient modulation scheme to increase the bandwidth efficiency and prevent the inter-symbol interference between signals. The OFDM technology has been widely applied to various wired and wireless communication systems, such as ADSL, 3GPP-LTE (Long Term Evolution) and WiMAX (Worldwide Interoperability for Microwave Access).
Refer to FIGS. 1A and 1B for a two-phase and half-duplexing OFDM AF relaying system. In the first phase, the source node transmits signals, and the relay node receives signals. In the second phase, the signals are amplified by the relay node and are transmitted to the destination node. Two cooperative protocols are involved in the system: the RD (Received Diversity) protocol and the MH (Multi-Hop) protocol. Under the RD protocol, the destination node receives the signals respectively from the source node and the relay node in the first phase and the second phase. Under the MH protocol, the destination node only receives the signals from the relay node in the second phase.
Under the above-mentioned RD protocol, the relaying system is used to promote diversity. Owing to the transmission characteristic of wireless channels, the destination node receives signals from the source node through a direct path (source-to-destination) and signals from the relay node through a relay path (source-to-relay-to-destination). The signals (carrying the same data) from different paths are performed diversity combining at the destination node to obtain diversity, whereby to reduce the shadowing effect and multi-path fading and improve the signal quality at the destination node.
Currently, the channel estimation methods for the OFDM-based AF relaying system have the following related technologies. C.S. Patel and GL. Stuber disclosed an LMMSE (Linear Minimum Mean Square Error) channel estimation method in a paper “Channel Estimation for Amplify and Forward Relay Based Cooperation Diversity Systems”, IEEE Trans. Wireless Commun., Vol 6, pp. 2348-2356, 2007. Fand Liu, Zhe Chen, Xin Zhang and Dacheng Yang disclosed a low rank MMSE channel estimation method in a paper “Channel Estimation for Amplify and Forward Relay in OFDM System”, International Conference on Wireless Communications, Networking and Mobile Computing, October 2008, pp. 1-4, which is based on an SVD (Singular Value Decomposition) method and exempted from inverse operation of the channel correlation matrix.
The above-mentioned prior arts focus on the estimation of the composite channel (source-to-relay-to-destination) coefficient rather than the estimation of the respective channel coefficients of source-to-relay and relay-to-destination in the relay path. However, in order to obtain diversity at the destination node, the estimation of two channels of the source-to-relay and the relay-to-destination are required in diversity combining. The difficulty of estimating individual channel is that the signal amplified by the relaying system and transmitted to the destination node no more makes the channel and noise observed at the destination node to be presented in a Gaussian distribution. This thus leads all the current AF relaying systems in the prior arts to estimate channels in a suboptimal way. Further, in the conventional literatures, the channel estimation methods are based on MIP (Multipath Intensity Profile), which is obtained by additionally consuming system resources. The present invention is exempted from using MIP.